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Abstract:

The invention includes: applying an anisotropic conductive resin
including conductive particles only to a plurality of bumps of an
electronic component; placing the electronic component above a main
surface of a flexible wiring board via the anisotropic conductive resin;
and pressurizing the electronic component to the wiring board and curing
the anisotropic conductive resin applied to the plurality of bumps to
join the plurality of bumps to the electrodes of the wiring board. This
can prevent a defective mounting of the electronic component.

Claims:

1-7. (canceled)

8. A circuit board comprising: a flexible wiring board; an electronic
component mounted on a main surface of the wiring board via a plurality
of bumps; first resin layers that are made of anisotropic conductive
resin including conductive particles and that individually cover the
plurality of bumps; and a second resin layer that fixes the electronic
component and the wiring board.

9. The circuit board according to claim 8, wherein, the first resin
layers are provided at inner sides of edges of the electronic component.

10. The circuit board according to claim 8, wherein, the electronic
component is a storage element that stores predetermined information; and
the wiring board includes a wireless communication antenna that is
electrically connected to the electronic component and that is used to
read the predetermined information stored in the electronic component.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a circuit board in which an
electronic component is mounted to a flexible wiring board and a method
for producing the circuit board.

BACKGROUND ART

[0002] Conventionally, one method has been known to mount an electronic
component on a wiring board to produce a circuit board. According to this
method, a bump is provided on an electrode of an electronic component to
join the bump with the electrode on the wiring board. For example, a
method has been suggested to transfer electrically-conductive adhesive
agent onto electric connection contact points (bumps) of a semiconductor
chip by the stamping method for example to heat the conductive adhesive
agent while pressing the semiconductor chip to a circuit board (e.g., see
Patent Publication 1).

[0003] On the other hand, an Integrated Circuit Card (IC card) has been
widely used in recent years as a card-type recording medium substituting
a magnetic card. A method for mounting a bare chip on an IC card board
has been suggested according to which a bare chip having a gold bump is
flip-chip-mounted via silver paste or an anisotropic conductive film
(e.g., see Patent Publication 2). A technique also has been known that a
tip end of a protruded electrode (bump) of an electronic component is
caused to bite into an electrode of an IC card board for mounting to use
thermoplastic resin to seal the electronic component on the board (e.g.,
see Patent Publication 3).

[0004] In the case of Patent Publication 1 however, the smaller size the
mounted electronic component has, the shorter distance between bumps is
caused. Due to this reason, when the electronic component is mounted via
conductive adhesive agent having a high filler content rate (e.g., silver
paste), the conductive adhesive agent is expanded between the bumps,
which may cause a defective mounting between the electrodes (e.g., short
circuiting). Also due to this reason, when the electronic component is
mounted via nonconductive resin paste for example, limitation is caused
on the reliability of the electric connection between the bump and the
electrode.

[0005] The IC cards disclosed in Patent Publications 2 and 3 generally use
a wiring board made of polyethylene terephthalate (PET) for the purpose
of realizing a thinner thickness and reduced cost. However, since the
PET-made wiring board is highly-flexible, the electronic component
mounted above the wiring board causes the deflection and deformation of a
part of the wiring board in the vicinity of a region at which a bump is
pressed. Thus, the lower surface of the electronic component is close to
the upper surface of the wiring board at positions in the vicinity of the
edges of the electronic component and a position near the center between
the bumps. When the electronic component is electrically-connected to the
wiring board via anisotropic conductive resin for example in this case,
conductive particles included in the anisotropic conductive resin are
sandwiched between the electronic component and the wiring board at a
part at which the electronic component is close to the wiring board. This
consequently leads to electrical connection between the electronic
component and the wiring board, resulting in a disadvantageous defective
mounting of the electronic component for example.

[0006] A method for producing a circuit board of the present invention
includes: a step a) of applying anisotropic conductive resin including
conductive particles only to a plurality of bumps of an electronic
component; a step b) of placing the electronic component above a main
surface of a flexible wiring board via the anisotropic conductive resin;
and a step c) of pressurizing the electronic component to the wiring
board and curing the anisotropic conductive resin applied to the
plurality of bumps to join the plurality of bumps to a wiring of the
wiring board.

[0007] Since this method applies the anisotropic conductive resin only to
the plurality of bumps, this method can produce a circuit board for which
a defective mounting of the electronic component is prevented.

[0008] The circuit board of the present invention includes: a flexible
wiring board; an electronic component mounted on a main surface of the
wiring board via a plurality of bumps; first resin layers made of
anisotropic conductive resin including conductive particles individually
covering the plurality of bumps; and a second resin layer that fixes the
electronic component and the wiring board.

[0009] This configuration can stably fix the electronic component to the
wiring board, thus realizing a circuit board that can be reliably
connected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a top view illustrating the configuration of a circuit
board according to an exemplary embodiment of the present invention.

[0011]FIG. 2 is a cross-sectional view illustrating a part of the circuit
board according to the exemplary embodiment of the present invention
taken along the line 2-2 of FIG. 1.

[0012]FIG. 3 is a flowchart illustrating the flow of the production of
the circuit board according to the exemplary embodiment of the present
invention.

[0013]FIG. 4A is a partial cross-sectional view illustrating a method for
producing the circuit board according to the exemplary embodiment of the
present invention.

[0014]FIG. 4B is a partial cross-sectional view illustrating the method
for producing the circuit board according to the exemplary embodiment of
the present invention.

[0015]FIG. 4C is a partial cross-sectional view illustrating the method
for producing the circuit board according to the exemplary embodiment of
the present invention.

[0016] FIG. 4D is a partial cross-sectional view illustrating the method
for producing the circuit board according to the exemplary embodiment of
the present invention.

REFERENCE MARKS IN THE DRAWINGS

[0017] 1 Circuit board

[0018] 2 Wiring board

[0019] 3 IC chip (electronic component)

[0020] 4 First resin layer

[0021] 4a, 40 Anisotropic conductive resin

[0022] 5 Second resin layer

[0023] 5a Adhesive agent

[0024] 21 Upper surface

[0025] 22 Antenna

[0026] 23 and 32 Electrode

[0027] 31 Chip body

[0028] 33 Bump

[0029] 42 Member

[0030] 91 Chip retainer

[0031] 92 Pressurizing tool

[0032] 311 Lower surface

[0033] 331 Protruded section

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034]FIG. 1 is a top view illustrating the configuration of a circuit
board according to an exemplary embodiment of the present invention. FIG.
2 is a cross-sectional view illustrating a part of the circuit board
according to the exemplary embodiment of the present invention taken
along the line 2-2 of FIG. 1.

[0035] As shown in FIG. 1 and FIG. 2, circuit board 1 is an inlet for an
Integrated Circuit Card (IC card) for example. Circuit board 1 includes:
flexible wiring board 2 made of PET for example; electronic component 3
such as an IC chip (hereinafter referred to as "IC chip") mounted on one
main surface 21 of wiring board 2 (which is the upper main surface in
FIG. 2 and hereinafter referred to as "upper surface" hereinafter); and
antenna 22 that sends and receives information or electric power. As
shown in FIG. 2, wiring board 2 is generally mounted in such a manner
that wiring board 2 is bent in the vicinity of parts at which wiring
board 2 is joined to IC chip 3. It is noted that the deflection of wiring
board 2 shown in FIG. 2 is exaggerated. IC chip 3 shown in FIG. 1 and
FIG. 2 is a storage element for storing predetermined information (e.g.,
product history, control data) and is a Large Scale Integration (LSI)
including a Ferroelectric Random Access Memory (FeRAM) for example.

[0036] Chip body 31 of IC chip 3 has a substantially-rectangular shape
when seen from the top. A surface of chip body 31 opposed to wiring board
2 (hereinafter referred to as "lower surface 311") has electrodes 32 on
which bumps 33 are provided. IC chip 3 is mounted on upper surface 21 of
wiring board 2 via bumps 33. As shown in FIG. 2, bump 33 is a stud-like
ball bump (so-called stud bump) that has protruded section 331 at the tip
end thereof for example.

[0037] As shown in FIG. 1, antenna 22 for wireless communication is formed
to have a coil-like shape along the outer periphery of wiring board 2.
Antenna 22 is electrically connected to IC chip 3 via two electrodes 23
that are formed on upper surface 21 of wiring board 2 and that constitute
a part of the wiring configuring antenna 22. Antenna 22 reads the
information stored in IC chip 3. Two bumps 33 positioned at the left side
of circuit board 1 in FIG. 1 are electrically connected to two electrodes
23 on wiring board 2, respectively. Bump 33 positioned at the right side
of FIG. 1 is a dummy bump for realizing the stable mounting of IC chip 3
for example and is not electrically connected to wiring board 2 in this
case.

[0038] As shown in FIG. 1 and FIG. 2, circuit board 1 includes first resin
layers 4 that are provided at the inner sides of the edges of chip body
31 of IC chip 3 and that individually cover bumps 33. First resin layer 4
is made of anisotropic conductive resin including minute conductive
particles for example.

[0039] As shown in FIG. 1, circuit board 1 includes second resin layer 5
that is provided at least between upper surface 21 of wiring board 2 and
lower surface 311 of IC chip 3 to cover first resin layers 4 and that
adheres IC chip 3 to wiring board 2. Second resin layer 5 is made of
adhesive agent that is nonconductive resin for example. For easy
understanding, FIG. 1 does not show second resin layer 5.

[0040] Wiring board 2 preferably has a thickness of 5 μm or more and 50
μm or less. In this exemplary embodiment, wiring board 2 has a
thickness of about 12.5 μm. IC chip 3 has a thickness of about 50
μm for example. Bumps 33 have a height of about 25 μm. Wiring board
2 having a thickness smaller than 5 μm is difficult to be handled and
wiring board 2 having a thickness exceeding 50 μm makes it difficult
to realize a circuit board that is thinner and that is flexible.

[0041] The following section will describe a method for producing circuit
board 1 according to the exemplary embodiment of the present invention
with reference to FIG. 3. FIG. 3 is a flowchart illustrating the flow of
the method for producing the circuit board according to the exemplary
embodiment of the present invention. FIG. 4A to FIG. 4D are a partial
cross-sectional view illustrating the method for producing circuit board
1. It is noted that FIG. 4A to FIG. 4D are a partial cross-sectional view
taken along line 2-2 in FIG. 1.

[0042] First, as shown in FIG. 4A, chip retainer 91 of a mounting
apparatus (not shown) is used to retain IC chip 3 to abut at least
protruded sections 331 at the tip ends of bumps 33 of IC chip 3 against
layer-like anisotropic conductive resin 40 formed on member 42 having a
predetermined smooth surface. Thereafter, IC chip 3 is lifted to separate
bumps 33 from layer-like anisotropic conductive resin 40. As a result, as
shown in FIG. 4B, anisotropic conductive resin 4a including minute
conductive particles are applied (or transferred) to at least protruded
sections 331 at the tip ends of bumps 33 (Step S11).

[0043] Next, as shown in FIG. 4B, adhesive agent 5a such as nonconductive
resin paste is applied to a region other than regions to be joined with
bumps 33 (electrodes 23) among a region on upper surface 21 of wiring
board 2 on which IC chip 3 is to be mounted (a region at least opposed to
the lower surface of IC chip 3) (Step S12). Thereafter, chip retainer 91
is moved relative to the upper side of wiring board 2 to adjust the
position of IC chip 3 to wiring board 2 (Step S13). As a result, two
bumps 33 other than the dummy bump are opposed to two electrodes 23
constituting a part of wiring board 2, respectively.

[0044] Next, as shown in FIG. 4C, IC chip 3 and chip retainer 91 are
lowered together to place IC chip 3 above upper surface 21 of wiring
board 2 via anisotropic conductive resin 4a applied to protruded sections
331 at the tip ends of bumps 33. At the same time, adhesive agent 5a is
caused by the own weight of chip body 31 to be expanded over lower
surface 311 of chip body 31 to the periphery and is filled between IC
chip 3 and wiring board 2 (Step S14).

[0045] Next, as shown in FIG. 4D, the retention of IC chip 3 by chip
retainer 91 is cancelled to press IC chip 3 toward upper surface 21 of
wiring board 2 via pressurizing tool 92. Consequently, at least protruded
sections 331 of bumps 33 of IC chip 3 are abutted against electrodes 23
of wiring board 2 to establish the connection therebetween. At the same
time, wiring board 2 is bent by the pressing force of IC chip 3 and parts
of wiring board 2 in the vicinity of bumps 33 are deformed to the lower
side for example and surrounding parts slightly away from bumps 33 of
circuit board 2 are deformed to the upper side for example and are close
to lower surface 311 of IC chip 3. Wiring board 2 is particularly close
to IC chip 3 at positions in the vicinity of the edges of chip body 31 of
IC chip 3 and at a position near the center between bumps 33 (a position
near the center of a straight line connecting two neighboring bumps 33).

[0046] Anisotropic conductive resin 4a on wiring board 2 is expanded by
the pressurization of IC chip 3 to the peripheries of bumps 33 and
adhesive agent 5a is also further expanded. It is noted that anisotropic
conductive resin 4a applied in Step S11 to protruded sections 331 at the
tip ends of bumps 33 is in a small amount and thus anisotropic conductive
resin 4a expanded by bumps 33 is prevented from being expanded over the
edges of chip body 31 of IC chip 3 and thus remains in the vicinity of
bumps 33 between chip body 31 and wiring board 2. As a result, bumps 33
are connected to electrodes 23 in abutment thereto to crush the
conductive particles in anisotropic conductive resin 4a to secure the
electrical connection therebetween.

[0047] Simultaneously with the pressurization of IC chip 3 to wiring board
2, a heater (not shown) provided in pressurizing tool 92 is used to heat
IC chip 3 via pressurizing tool 92 while IC chip 3 being pressurized to
wiring board 2. Then, anisotropic conductive resin 4a applied to bumps 33
of IC chip 3 thermally-cures. As a result, as shown in FIG. 2, first
resin layers 4 individually covering bumps 33 are formed at the inner
sides of the edges of IC chip 3.

[0048] In this manner, the pressurization and heating of IC chip 3
establish the electrical connection between IC chip 3 and electrodes 23
of antenna 22 of wiring board 2 (see FIG. 1) via bumps 33 and also allow
IC chip 3 to be joined and mounted to wiring board 2. At the same time,
adhesive agent 5a thermally-cures to form second resin layer 5 that seals
the space between IC chip 3 and wiring board 2 to stably fix IC chip 3 to
wiring board 2 (Step S15). Through the above process, IC chip 3 is
mounted on wiring board 2 to produce circuit board 1.

[0049] Thereafter, the main surfaces at both sides of circuit board 1 are
covered by a cover sheet made of polycarbonate for example, thereby
producing an IC card. The resultant IC card has a thickness of about 0.76
mm for example.

[0050] As described above, according to circuit board 1 of this exemplary
embodiment, only a small amount of anisotropic conductive resin 4a is
applied to at least protruded sections 331 at the tip ends of bumps 33 of
IC chip 3. This can prevent, when IC chip 3 is mounted, conductive
particles included in anisotropic conductive resin 4a from being expanded
to the outer sides of the edges of chip body 31 of IC chip 3 or the
neighborhood of the center between bumps 33. In other words, conductive
particles can be enabled to exist only in the vicinity of a part at which
bumps 33 are joined to wiring board 2 and the neighborhood of this joint
part. Furthermore, according to circuit board 1 of this exemplary
embodiment, conductive particles can be prevented from reaching a part at
which a short distance is caused between upper surface 21 of wiring board
2 and lower surface 311 of IC chip 3. This can consequently prevent this
part from having the continuity between wiring board 2 and chip body 31
or the continuity between bumps 33, thereby preventing a defective
mounting of IC chip 3 (e.g., short circuiting) to realize circuit board 1
that can be reliably connected for example.

[0051] Also according to the method for producing circuit board 1 of this
exemplary embodiment, anisotropic conductive resin 4a applied to bumps 33
forms first resin layers 4 that are provided at the inner sides of the
edges of IC chip 3 and that individually cover bumps 33. This prevents
anisotropic conductive resin 4a from reaching the edges of IC chip 3 or
the center between bumps 33. This can more securely prevent conductive
particles from reaching such parts, thus more securely preventing a
defective mounting of IC chip 3. Due to the above reason, the method for
producing circuit board 1 of this exemplary embodiment is suitably used
to mount an electronic component such as IC chip 3 to wiring board 2 that
has a thin thickness of 5 μm or more and 50 μm or less and that is
easily deflected for example.

[0052] Also according to this exemplary embodiment, bumps 33 of IC chip 3
can be abutted against layer-like anisotropic conductive resin 40 to
easily apply anisotropic conductive resin 4a to protruded sections 331 of
bumps 33, thus realizing an improved productivity of circuit board 1.
Furthermore, bumps 33 of IC chip 3 formed as stud bumps having protruded
sections 331 at the tip ends thereof can use the bumpy shape of the tip
ends of bumps 33 to securely apply and retain anisotropic conductive
resin 4a only in an amount required to seal bumps 33.

[0053] Also according to this exemplary embodiment, second resin layer 5
can stably fix IC chip 3 to wiring board 2 and can seal the space between
chip body 31 and wiring board 2 to more securely prevent a defective
mounting of IC chip 3, thus providing higher reliability to circuit board
1.

[0054] The circuit board of this exemplary embodiment is particularly
advantageous in that chip body 31 of IC chip 3 does not require a
continuity-preventing processing by the formation of an insulating film
for example, thus preventing the communication characteristic of the IC
card from being influenced by such a processing. Therefore, the circuit
board of this exemplary embodiment is particularly suitable for a circuit
board such as an inlet of an IC tag using a wiring board including an
antenna for example.

[0055] As described above, the exemplary embodiment of the present
invention has been described. However, the present invention is not
limited to the above exemplary embodiment and various changes as shown
below are possible.

[0056] Specifically, although the above exemplary embodiment has described
an example in which bumps 33 of IC chip 3 are abutted against layer-like
anisotropic conductive resin 40 to apply anisotropic conductive resin 4a
to protruded sections 331 at the tip ends thereof, the present invention
is not limited to this. For example, a dispenser or the like also may be
used to sequentially apply anisotropic conductive resin 4a to the tip
ends of bumps 33.

[0057] Although the above exemplary embodiment has described an example to
apply adhesive agent 5a to a part other than electrode 23 of wiring board
2, the present invention is not limited to this. For example, adhesive
agent 5a also may be applied to surround a plurality of regions to be
joined with bumps 33 among regions of upper surface 21 of wiring board 2
on which IC chip 3 is to be mounted. In this case, adhesive agent 5a is
preferably not formed at a part of the entire periphery in order to
realize ventilation.

[0058] Although the above exemplary embodiment has described an example of
an inlet of an IC card, the present invention is not limited to this. For
example, the present invention also can be used to produce an inlet of an
IC tag for example and also can be used to mount a driver IC for a
display for example based on the chip-on-film (COF) method.

[0059] Although the above exemplary embodiment has described IC chip 3
such as a storage element as electronic component 3 mounted to wiring
board 2;

[0060] the present invention is not limited to this. For example,
electronic component 3 mounted to wiring board 2 also may be a mold-type
IC chip having bumps as in the chip size package (CSP) or a passive chip
component. Alternatively, a plurality of electronic components also may
be mounted.

[0061] Although the above embodiment has described an example of wiring
board 2 made of PET, the present invention is not limited to this. For
example, wiring board 2 also may be a flexible board made of polyether
ether ketone (PEEK) or polyimide for example.

INDUSTRIAL APPLICABILITY

[0062] The present invention can be used for a circuit board used as IC
card or an inlet of an IC tag or various other circuit boards.

Patent applications by Daido Komyoji, Nara JP

Patent applications by Hidenobu Nishikawa, Nara JP

Patent applications in class With particular conductive material or coating

Patent applications in all subclasses With particular conductive material or coating